In handling and storing bulk solids, many applications of tableting, particle size reduction, mixing, packaging and quality control require that bulk solid handling systems be designed to precise engineering specifications to ensure a reliable handling system. To design a reliable handling system, it is necessary to characterize the flow properties of the bulk solid to be used in the system. It is known to use a Yield Locus plot to evaluate the flow properties of bulk solids, especially powders. The Yield Locus is the plotting of an averaged straight line which is the average of several plotted points on a shear stress vs. normal stress plot. The plotted points of the Yield Locus represent different normal load stress states of material samples having the same initial bulk density and testing conditions.
Shear tests are common in powder technology to characterize the flowability of granular materials and powders. Several shear testers and procedures have been used to measure the flow properties of powders. Some of the commonly used testers are the Jenike shear cell, the direct shear box, the rotational shear cell, and the biaxial tester. Each tester has its own specific problems, but most of the testers mentioned require laborious test procedures and separate tests to obtain each point to be plotted on the Yield Locus plot. It has been found that no single tester is suitable for reliably testing all bulk solid materials.
The Jenike shear cell tester is the more widely used tester in industry. The Jenike tester is usually made up of an upper ring, lower ring, a base, a mold ring and a lid. The lower ring is fixed to the base and the upper ring is placed on top of the lower ring to form a shear cell. The molding ring removably interlocks on top of upper ring. The lid fits from above into the openings of the molding ring and upper ring. Tests with the Jenike tester are normally performed in two main stages. In the first stage, the shear cell is careful filled above the top of upper ring with the material to be tested. The molding ring retains the material that would normally flow over the top of the upper ring. The lid is then placed into the opening of the molding ring. Next, the material is initially compressed or consolidated by twisting the lid while applying a static axial load downward on the lid that is normal to the cell. After the initial consolidation, the axial load, lid and molding ring are removed. Any remaining powder the extends above the upper ring is scrapped away. Next, the lid is place back into the upper ring with the axial load. The axial load on the lid creates a vertical consolidation stress .sigma..sub.pr, also referred to as normal stress or normal load. The material is then forced to shear by horizontal displacement of the upper ring over the lower ring until a steady state failure condition is reached. A load cell is connected to whatever mechanism is employed to displace the upper ring. The load cell is used for measuring the shear stress T.sub.pr. The steady state failure is marked by the shear stress T.sub.pr, remaining constant. After reaching the steady state failure, the vertical normal stress is reduced to zero and the lid is retracted. In the second stage, a vertical normal stress of .sigma..sub.s is applied to the lid, whereby the value of .sigma..sub.s is less than the value of .sigma..sub.pr. The material is again forced to shear by horizontal displacement of the upper ring over the lower ring until the material shears to failure. The shear to failure this time is marked by the shear stress passing through a maximum value followed by a reduction in value. This maximum stress value from the second stage is one point to be plotted on the Yield Locus plot. In order to measure other points for the Yield Locus, the two stages must be repeated. When the stages are repeated for each point, a new unconsolidated sample of material must be loaded into the cell, whereby the same value of .sigma..sub.pr must be obtained, but a different value of .sigma..sub.s is applied for each point. As is evident, it is necessary to use a large amount of test material which may not be readily available in order to obtain a Yield Locus plot. Also, it is very difficult to reproduce the exact consolidation of the material to get the same value for .sigma..sub.pr and therefore a margin of error exists in the testing of a material.
There has also been some work done towards making test procedures simpler and faster with the development of testers which measure the Yield Locus from a single test. Such testers have been referred to as "constant volume" testers. However, published results either do not include comparisons with accepted standards, or the methods published still involve complicated test procedures. Therefore, there is a definite need to improve upon available testers and test procedures.
It is an object of the present invention to provide tester for bulk solids, especially powders, with which the Yield Locus data can be obtained directly from a single test using a simplified testing procedure.
It is another object of the present invention to provide a tester and a simplified testing procedure for reducing the amount of material needed to be tested in order to obtain a Yield Locus plot.